Method for coating an indirectly heated cathode



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July 26, 1966 Filed Dec.

H. PROVISOR 3,262,814

METHOD FOR COATING AN INDIRECTLY HEATED CATHODE 2 Sheets-Sheet 2 COAT INNER SURFACE OE NICKEL SLEEVE WITH MIXTURE OF NiO AND Al HEAT COATED SLEEVE 'IO 590C TO CONVERT MIXTURE] T0 GREY-BLACK COATING HEAT GREY-BLACK COATED SLEEVE IN HUMID, REDUCING ATMOSPHERE AT 550C 'IO 600C FOR 30 MINUTES TO FORM STABLE COMPOUND WITH UNBOUND Al COATING TO OF SLEEVE APPLY EMISSIVE OUTER SURFACE MOUNT E'ILAMENT WITHIN SLEEVE FIG. 2

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AGE/Y7 United States Patent Filed Dec. 14, 1961, Ser. No. 159,255 Claims priority, application France, Dec. 15, 1960, 847 62 ,0 2 Claims. (Cl. 117227) This invention has .for its object to provide improved indirectly heated cathodes, of which the electron-emissive surface is formed by an alkaline-earth oxide layer or by a layer containing such oxides.

The improvement obtained by this invention relates to furthering the heat transfer from the filament, which constitutes the source of heat, to the cathode proper, i.e. to the part serving as a carrier for the emissive layer across the space separating and insulating these two ele ments. The invention relates mainly to the application of a black layer to the surface of the said carrier body, exposed to the direct heat radiation from the said filament, in order to raise the absorption power of this surface for the said radiation.

The invention furthermore relates to a method of applying this black layer and, moreover, to electron tubes comprising cathodes provided with such a black layer.

It is known that with the same type of electron tubes a higher temperature of the filament body during operation brings about a correspondingly higher risk of parasitic currents or of a short-circuit between the filament and the cathode, so that the tube may become defective. In general, it is assumed that the risk of short-circuit or break-down between filament and cathode is an exponential function of the temperature of the filament. It is furthermore known that breakdown is one of the most frequent causes shortening the lifetime of electron tubes.

It has been found that, in order to prolong the lifetime of such a tube, the temperature of the filament must be kept as low as possible for maintaining a predetermined temperature of the emissive layer. Use must therefore be made of means for transferring to the cathode proper a maximum quantity of the heat produced by the filament.

Since between the cathode and the filament a small, empty space exists, within which the heat can be transferred only by radiation, the aforesaid object can be realized only by utilizing the radiation properties of dark bodies. Such means therefore consist in blackening the insulating coating of the filament and the inner surface of the cathode to the maximum extent.

The object of the invention is attained, when the filament has a minimum temperature with a given tempera ture of the emissive layer, so that the temperature difference between the filament and the emissive layer is at a minimum.

It has been found that if the surface of a cathode facing the filament is blackened, the temperature difference between the cathode and filament is decreased. Consequently, the breakdown resistance between the cathode and the filament is materially increased.

The present invention has for its object to apply a black layer to the surface of the cathode proper, i.e. the supporting body of the emissive layer, exposed directly to the radiation from the filament. In accordance with the invention this layer on the said surface is formed by the reaction of aluminum in extremely fine state with the cathode metal at a temperature of about 600 C. in a reducing or a neutral atmosphere or in vacuo. The reaction yields a grey-black intermetallic layer. The metal of the supporting body of the emissive layer may consist of Patented July 26, 1966 nickel, molybdenum, iron or an alloy having one of these metals as a base.

In accordance with the type of electron tube in which the cathode is to be employed, this cathode may be tubular (for example a hollow cylinder) or flat (for example a strip). The invention therefore relates to two shapes of the black layer on the cathode metal.

In a first embodiment of the invention the aluminum required for the reaction is provided in the form of a fine powder suspended in a fluid. The suspension is applied to the surface of the carrier body to be blackened. By drying in air an aluminum powder layer with an adhesive formed by the suspension agent, is formed on the body. If the carrier body is tubular, the suspension is introduced into the tube, for example by sucking. If the body is strip-shaped, the suspension may be applied by atomizing.

An example of a suspension which yields satisfactory results, is a solution of 2.50 gms. of nitrocellulose in gms. of ethylene-glycol, and 22.5 gms. of butanol; in mls. of this solution 20 gms. of fine aluminum powder is suspended.

If the metal of the carrier body is nickel, an addition of nickel oxide to this suspension may provide a considerable improvement in the uniformity of the black layer; 5 to 15 gms. of nickel oxide is a suitable quantity with 20 gms. of aluminum powder.

The suspension composition given above is, of course, only stated by way of example; the ratio between the suspension liquid and the aluminum powder and the nickel oxide may be varied within Wide limits. Even the suspension agent may be differently composed. A solution of methacrylate, for example methylmethacrylate, may replace the solution of nitro-cellulose. The suspension agent need serve only as a medium for the aluminum and as a provisional adhesive for sticking the aluminum particles to each other and to the metal surface. After the reaction of the aluminum with the metal of the surface no residues should be left which are likely to change the electronic properties of the cathode.

In a second embodiment of the invention the aluminum required for the reaction is applied by condensing aluminum vapour, which is obtained in vacuo. This method is particularly suitable for a strip-shaped metal carrier body. The surface of the strip is covered with an aluminum layer of a thickness of a fraction of a micron; the layer is produced by the thermal evaporation of aluminum in vacuo.

The further treatment according to the invention is the same in the two methods described above. The coated metal supporting body is sintered in vacuo, in a neutral or in a reducing atmosphere. Satisfactory results are obtained with a temperature lying between 610 C. and 650 C. during 15 to 30 minutes. The sintering temperature should not be lower than 590 C.; it may, 'however, exceed 650 C. The sintering period may vary between a few minutes and a few hours, if it is found to be necessary.

For a practical use of this method for blackening cathodes it should be noted that with the conventional cylindrical nickel cathodes, sintering does not adversely affect the rigidity, so that during mounting of the cathodes no difiiculties arise. Sintering for 30 minutes at 610 C. in vacuo with circular, tubular cathodes having a wall thickness of 60 and a length of 2.5 cms. provides a rigidity of about 400 gms. as compared with an initial rigidity of 500 gms. The rigidity is measured in this case by determining the load required for obtaining a given bend in the center of a test body during a bending test. It is furthermore known that aluminum, dissolved in an adequate quantity in nickel, produces within a short time a parasitic impedance on the interface between the alkaline earth metal oxide and the nickel. In the present case, a small quantity of aluminum diffuses, it is true, rapidly across the nickel surface and thus reaches this interface, but tests have shown that the emission of the cathode remains constant and that no parasitic impedance of the cathode occurs within 2500 operational hours of the tube, even if the voltage of the filament exceeds by 15% that used in normal operation. After the said period a given amount of parasitic impedance may be ascertained, but this is mainly due to the disengagement of the oxide layer.

The aluminum blackening of the inner surface of the cathode is not advisable for tubes having a long lifetime. The undesirable diffusion of aluminum may, however, be avoided to a considerable extent by oxidizing part of this metal subsequent to sintering. A second sintering treatment between 550 C. and 600 C. for 30 minutes in a humid, reducing atmosphere does not change considerably the rigidity of the parts and converts the aluminum not yet bound into a comparatively stable compound. The coating thus formed maintains the absorption power for the radiation heat obtained in the first layer of the intermetallic compound.

The invention will now be described more fully with reference to the accompanying drawing.

FIG. 1 shows, by way of example, a longitudinal sectional view of a cylindrical cathode according to the invention.

FIG. 2 shows the process of manufacturing a cathode according to the invention.

The cathode shown in FIG. 1 comprises a filament body 1, which is accommodated in a central, refractory body 4 shaped in the form of a rod, hereinafter termed heating body. The heating body 4 serves at the same time as a support and an insulator for the filament 1. The heating body 4 is coated on the outer side with a grey-black layer, obtained by the aforesaid reaction of aluminum and nickel. A tubular envelope 5 of nickel is arranged concentrically around the heating body 4. The inner surface of this envelope is coated with a black layer 6 of the inter-metallic aluminum-nickel compound obtained by carrying out one of the aforesaid methods. The said envelope is provided on the outer surface with an emissive layer of suitable thickness 7, which contains mainly barium oxide and strontium oxide.

The efficiency of the grey-black layer obtained in the manner described above with respect to the heat exchange between the filament and the cathode can be easily checked by measuring the temperature of these two elements, after they have been arranged in an exhausted bulb. With a filament voltage of 6.3 v. at the filament arranged in a blackened cathode envelope a temperature drop of about 100 C. is measured with respect to a conventional filament in a conventional cathode envelope. The blackened envelope is, moreover, considerably hotter than the conventional cathode.

The influence of such a blackening on the lifetime of tubes may be proved by the following test. In accordance with the first-mentioned method of the invention, cathode envelopes are blackened for use in a diode, which is extremely sensitive to break-down between the cathode and the filament; the diodes are subjected to an endurance test in a test arrangement. A sequence of ten tubes, comprising these cathode envelopes and a comparison sequence with the conventional cathode envelopes were used. The tubes were driven for a long time under the following operational conditions: the voltage V at the filament 1 was 6.3 v.; the voltage V between the filament 1 and the cathode was v. the voltage V between the cathode and the anode was w The last two voltages were fed via a resistor of 15.000 ohms, connected in series with the cathode; this resistor served as a current limiter in the event of a break-down. This resistor was shunted by a capacitor C of 10 ,uf. None of the comparison diodes ope-rated for a longer period than 500 hours, since a break-down occurred between the cathode and the filament body. However, even after 2500 hours no shortcircuit and no abnormal phenomena occurred in the said tubes comprising the blackened cathode envelopes.

As a matter of fact, within the scope of the present invention variants of the methods described above may be used, if other equivalent technical means are employed.

What is claimed is:

1. A method of manufacturing an indirectly heated cathode including a heating element, a nickel supporting body in heat-transfer relationship to the heating element the surface of which facing the heating element being covered with a black layer, and an emissive layer on a surface of the supporting body on a side thereof remote from the heating element, which comprises the steps, forming on the surface of said supporting body facing the heating element a layer of finely divided aluminum and nickel oxide, heating the so-covered supporting body to a temperature of at least 590 C. in a vacuum to form a grey-black intermetallic layer, and heating the body with the grey-black layer in a humid, reducing atmosphere to a temperature between 550 C. and 600 C. for 30 minutes to convert unbound aluminum into a stable compound.

2. A method as claimed in claim 1, in which the body with the layer of finely-divided aluminum and nickel oxide is heated to a temperature lying between 610 C. and 650 C.

References Cited by the Examiner UNITED STATES PATENTS 2,723,363 11/1955 DeSantis et al 313-337 2,740,726 4/1956 Anderson 117-46 X 2,817,572 12/1957 Weber.

2,891,879 6/1959 Rohrer 117227 X 2,949,557 8/1960 Gerlach 313-346 2,987,423 6/1961 Sternberg 1l7-46 X 3,056,061 9/1962 Melert 3l3346 3,068,337 12/1962 Kuebrich 1171()7 X MURRAY KATZ, Primary Examiner.

BLYTHE B. MILLER, GEORGE N. WESTBY, RICH- ARD D. NEVIUS, Examiners.

L. D. BULLION, D. E. SRAGOW, J. P. MCINTOSH,

Assistant Examiners. 

1. A METHOD OF MANUFACTURING A INDIRECTLY HEATED CATHODE INCLUDING A HEATING ELEMENT, A NICKEL SUPPORTING BODY IN HEAT-TRANSFER RELATIONSHIP TO THE HEATING ELEMENT THE SURFACE OF WHICH FACING THE HEARING ELEMENT BEING COVERED WITH A BLACK LAYER, AND AN EMISSIVE LAYER OF A SURFACE OF THE SUPPORTING BODY ON A SIDE THEREOF REMOTE FROM THE HEATING ELEMENT, WHICH COMPRISES THE STEPS, FORMING ON THE SURFACE OF SAID SUPPORTING BODY FACING THE HEATING ELEMENT A LAYER OF FINELY DIVIDED ALUMINUM AND NICKEL OXIDE, HETING THE SO-COVERED SUPPORTING BODY TO A TEMPERATURE OF AT LEAST 590*C. IN A VACUUM TO FORM A GREY-BLACK INTERMETALLIC LAYER, AND HEATIING THE BODY 